This chair isn’t empty though… I don’t understand anything anymore. So she left, some seat-filler took her spot, and then he left too? Did he reappear somewhere else? Maybe with his crew (which would make sense for the last category, right?).

I think she left, then he left too and he came back alone and sat next to that lady on the picture, which confirms that Gillian didn’t come back or he would have just wait for her and kept her seat.
I wasn’t watching. But from the little sneak peaks, and the pictures I saw, and also from what people who watched told me, that’s what I think happened.